This invention relates to radiant energy collection and more particularly to a cavity design for reducing losses inherent in certain radiant energy collectors.
This invention relates to the subject matter of my copending patent application, Ser. No. 113,169; assigned to the same assignee and filed on even date herewith now abandoned, and superseded by continuation patent application Ser. No. 230,137, filed Jan. 30, 1981.
It is well-established that radiation which is partially collimated with an angular divergence of .+-..theta. can be concentrated on a receiver without transmission loss by the maximum concentration of C.sub.max=sin .theta..sup.-1 through the use of nonimaging reflectors known generically in the art as compound parabolic concentrators. These concentrators are disclosed in my U.S. Pat. Nos. 3,923,381; 4,002,499; and 4,033,638; the disclosures of which are incorporated herein by reference. The compound parabolic concentrator (CPC) includes an energy receiver positioned between two trough-like sidewalls which reflect substantially all incident energy received over a predetermined included angle (.+-..theta.) onto the energy receiver. The aforementioned patents describe how the reflective sidewalls should be configured to achieve the maximal energy concentration. For instance, in U.S. Pat. No. 4,002,499, it is shown, for a tubular energy receiver, that the lowermost portions of the sidewalls form an involute of the shape of the energy receiver. It is important to note that the energy receiver shown in the '499 patent is the theoretical "optical design" receiver, and a practical design dictates that a larger-than-theoretical receiver shape be emplaced between the sidewalls to ensure that placement tolerances, minor wall malformations, etc. do not significantly hinder the energy concentrator. More recently, it has been shown desirable to encompass the receiver in a vacuum bottle-like structure wherein the outermost surface is concentric to the inner receiver; is transparent; and encloses a vacuum between itself and the inner surface of the receiver (which itself may be coated with an energy selective surface).
To strictly meet the requirements of maximum concentration and no transmission loss in a CPC, the reflector surfaces should touch the optical design receiver. At times this is not practical--especially when a larger-than-theoretical receiver is employed or when the receiver is enclosed in a transparent vacuum jacket.
In fact, it is desirable in most constructions that the actual receiver be offset somewhat from the reflector walls to prevent mechanical interferences when they are assembled. This offset reduces the efficiency of the system.
W. R. McIntire of the Argonne National Laboratory has developed a cavity/reflector design for the region below a tubular absorber. The McIntire cavity, for gaps between the cavity and the absorber of up to approximately half of the optical receiver radius, eliminates some of the losses referred to above. That design is shown in FIG. 1 and utilizes a trough-shaped cavity 10 positioned beneath the optical receiver 12. The "W" design (four linear segments) of cavity 10 ensures that no energy rays can enter the region between receiver tube 12 and cavity 10 without being reflected onto receiver 12. R.sub.1 is the radius of the circular cross section of the optical receiver, and R.sub.2 is the maximum distance which cavity 10 can be separated from the center of optical receiver 12. McIntire shows that for the four-segment cavity 10, R.sub.2 should be no larger than .sqroot.2R.sub.1.
When a McIntire-type structure is combined with a CPC structure having sidewalls 14 and 16, its capability to concentrate energy is degraded somewhat in comparison to the cusp-type CPC concentrator shown in U.S. Pat. No. 4,002,499. This relationship is shown in FIG. 2 where is plotted the ratio of the cusp-type CPC concentrator to a CPC modified as shown in FIG. 1. Along the x axis is plotted the ratio of R.sub.2 /R.sub.1. The portion of curve 18 which is to the left of dotted line 20 shows the degradation of concentration for the reflector shown in FIG. 1 as cavity 10 is moved further away from optical receiver 12.
One problem with the four-segment cavity 10 is that it must be placed quite close to the optical receiver for it to accomplish its function. Since practical receiver structures are constructed considerably larger than their optical design size and are also often enclosed in vacuum jackets which further increase their diameter, that structure may not result in a practical solution.
It is therefore an object of this invention to provide an optical trough cavity which provides enhanced energy concentration ability.
It is a further object of this invention to provide an energy trough cavity/reflector which may be placed at a distance greater than .sqroot.2R.sub.1 away from an optical receiver.
It is still another object of this invention to provide an optical reflecting cavity which may be used either in conjunction with a CPC structure or with imaging-type structures.